1. Field of the Invention
The invention relates to an organic light emitting diode (OLED) display device, which may be referred to as an organic electroluminescent display device. More particularly, the invention to an OLED display device having a bank of a double-layered structure and a method of fabricating the same.
2. Discussion of the Related Art
An OLED display device of flat panel display devices has high brightness and low driving voltage. The OLED display device is a self-emitting type and has excellent characteristics of view angle, contrast ratio, response time, etc.
Accordingly, the OLED display device is widely used for a television, a monitor, a mobile phone, and so on.
The OLED display device includes an array element and an organic light emitting diode. The array element includes a switching thin film transistor (TFT), which is connected to a gate line and a data line, a driving TFT, which is connected to the switching TFT, and a power line, which is connected to the driving TFT. The organic light emitting diode includes a first electrode, which is connected to the driving TFT, and further includes an organic light emitting layer and a second electrode.
In the OLED display device, light from the organic light emitting layer passes through the first electrode or the second electrode to display an image. A top emission type OLED display device, where the light passes through the second electrode, has an advantage in an aperture ratio.
Generally, the organic light emitting layer is formed by a thermal deposition method using a shadow mask. However, the shadow mask sags because the shadow mask becomes larger with an increase in sizes of display devices. As a result, there is a problem in deposition uniformity in the larger display device. In addition, since a shadow effect is generated in the thermal deposition method using the shadow mask, it is very difficult to fabricate a high resolution OLED display device, e.g., above 250 PPI (pixels per inch).
Accordingly, a method different from the thermal deposition method using the shadow mask has been introduced.
In the method, a liquid phase organic light emitting material is sprayed or dropped in a region surrounded by a wall using an liquid-releasing apparatus or a nozzle-coating apparatus and cured to form the organic light emitting layer.
FIGS. 1A to 1C are schematic cross-sectional views showing an OLED display device according to the related art in steps of fabricating the OLED display device and steps of forming a bank and forming an organic light emitting layer by spraying or dropping a liquid phase organic light emitting material.
To spray or drop the liquid phase organic light emitting material by the liquid-releasing apparatus or the nozzle-coating apparatus, the bank, which is formed on a first electrode and surrounds a pixel region, is required to prevent the liquid phase organic light emitting material from flooding into a next pixel region. Accordingly, the bank is formed on edges of the first electrode before forming the organic light emitting layer.
At this time, the bank is formed of a material having a hydrophobic property. The hydrophobic bank prevents the liquid phase organic light emitting material, which has a hydrophilic property, from being formed on the bank and flooding into the next pixel region due to a mis-alignment of the liquid-releasing apparatus or the nozzle-coating apparatus or an excessive amount of the organic light emitting material.
As shown in FIG. 1A, the bank may be formed by a mask process, which includes a light-exposing step using an exposing mask 91 and a developing step after an organic insulating material having a hydrophobic property is applied to an entire surface of a substrate 10 on which a first electrode 50 is formed at each pixel region P.
Here, the light-exposing step using the exposing mask 91 may include irradiating high lux UV light of several hundred mW/cm2.
With an increase in a size of a display device, a substrate for the display device has been larger, and a scan-type exposure has been performed onto the large-sized substrate because it is not possible to expose a whole of the large-sized substrate to light at a time.
The scan-type exposure decreases the productivity per unit time as compared to a process of exposing the substrate to light at a time. To prevent the productivity from being decreased, in the scan-type exposure, high lux UV light of several hundred mW/cm2 instead of typical UV light of several dozen mW/cm2, which is generally used in a light-exposing step, has been used.
When the light-exposing step with the high lux UV light is performed, a scan speed is several times to several dozen times as fast as the typical UV light. Therefore, a process time of the light-exposing step per unit time decreases, and the productivity per unit time increases.
However, when the high lux UV light of several hundred mW/cm2 is irradiated to an organic insulating material layer 52 by a scan-type exposure apparatus 80 using the high lux UV light, light from the scan-type exposure apparatus 80 may be reflected by signal lines 30 or electrodes (not shown) of a metallic material or by a stage 93 of the exposure apparatus 80 and may reach a central portion of the pixel region P even if an amount of the light is a little. Here, the organic insulating material layer 52 has a hydrophobic property.
Accordingly, as shown in FIG. 1B, after developing the organic insulating material layer 52 of FIG. 1A exposed to light, the bank 53 is formed along boundaries of the pixel region P, and organic insulating material residues 54 with the hydrophobic property remain on the first electrode 50 at the central portion of the pixel region P. The organic insulating material residues 54 may be referred to as organic insulating material residual layers.
Meanwhile, the bank 53 has a width w1′ larger than a designated width w1 because a tail is lengthened due to the reflected light during the light-exposing step.
Next, as shown in FIG. 1C, by spraying or dropping a liquid phase organic light emitting material from an liquid-releasing apparatus 95 into the pixel region P, which is surrounded by the bank 53, the pixel region P is filled with the organic light emitting material. The organic light emitting material is dried and cured by heat to form an organic light emitting layer 55.
However, since the residues 54 have the hydrophobic property, the residues 54 hinder the liquid phase organic light emitting material from being spread in the pixel region P when the liquid phase organic light emitting material is sprayed or dropped. Accordingly, as shown in FIG. 1C and FIG. 2, which is a picture of showing one pixel region in the related art OLED display device, the organic light emitting layer 55 is not formed around the hydrophobic bank 53, or a portion of the organic light emitting layer around the hydrophobic bank 53 has a thinner thickness than portions in other regions. Thus, dark images are displayed in edges of the pixel region P. In addition, the OLED display device is degraded fast due to the difference in thicknesses, and the lifetime of the OLED display device is shortened.
Moreover, as stated above, since the bank 53 has the width w1′ larger than the designed width w1, an area for the organic light emitting layer 55 is reduced, and the aperture ratio is lowered.